Well, we've got an interesting challenge w/ Chandler. We'd need to renounce him (and others!) if we were to sign Carmelo as a FA. Alternatively, we'd probably need to pay him 6-9million per year, effectively eliminating our cap space.
Mayo still has a few years left potentially on his rookie deal.
I like the versatility that the Knicks current lineup brings - we have Gallo/Chandler/Amare/Fields - all are capable of playing multiple positions, allowing us to tweak our lineup for running or half court.
Gallo/Chandler each is not as good as a Carmelo - but a lineup with both creates opportunities a lineup with just Carmelo would. (I'm ambivalent on any move for Carmelo - though I'd be in favor of it strongly if we could keep at least one of Gallo/Chandler - both of them are turning into glue guys)
At this stage of their careers, Chandler is a better player than Mayo - clearly. You'd think Mayo has more upside/talent/bball IQ... but Chandler is "versatile," able to leverage strengths to his game whether he's playing SG/SF/PF. Mayo gets the negative connotation of tweener. He's a small SG... and hasn't shown the ability to exploit his size/skills against smaller opponents when playing PG.